1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device having an integrated circuit that uses metal electrodes and metal wiring lines. The invention also relates to a display device in which a pixel area having a matrix structure and driver circuits for driving switching elements which are arranged in the pixel area are formed monolithically.
2. Description of the Related Art
There are known active matrix liquid crystal display devices, which are characterized in that a number of pixels are arranged in a matrix of several hundred by several hundred in a pixel area, and a thin-film transistor is provided for each pixel to control charge entering or exiting the pixel electrode.
At present, driver circuits for driving the thin-film transistors arranged in the pixel area are constituted as an external IC circuit. The external IC circuit is formed on a single crystal wafer, because the peripheral driver circuits need transistor circuits having superior characteristics.
On the other hand, as a next-generation active matrix liquid crystal display device, there is now needed a configuration in which peripheral driver circuits and a pixel area are integrated on the same substrate (usually a glass substrate). In this case, it is necessary to constitute the peripheral driver circuits by using thin-film transistors formed on the glass substrate.
Further, in the active matrix liquid crystal display device incorporating the peripheral driver circuits in an integral manner, the area occupied by the peripheral driver circuits should be made as small as possible to minimize the area other than the pixel area.
Such reduced dimensions in design rules makes it more difficult to form wiring lines. Further, as the degree of miniaturization increases, the resistance of wiring lines themselves comes to be unnegligible. Therefore, it is necessary to use materials having as small a resistance as possible to form wiring lines, such as aluminum and a material mainly made of aluminum.
However, formation of wiring lines with a metal material mainly made of aluminum is associated with problems of deformation in wiring line shape and formation of a wiring line having an unintended shape due to abnormal growth of an aluminum component: hillocks, whiskers, and the like.
Hillocks and whiskers may be caused, for instance, by heating during film deposition, heating during resist ashing (i.e., resist removal by oxygen plasma), and heating due to illumination with laser light for annealing.
A hillock occurs due to abnormal growth of aluminum. More specifically, when partial abnormal growth of an aluminum component occurs, growing portions collide with each other to cause a mountainous protrusion. A whisker is a thorn-like or horn-like protrusion caused by abnormal growth of aluminum. While an exact cause of hillocks and whiskers is not known, it is believed to be a certain impurity in aluminum or non-uniformity in the aluminum crystal structure.
Since a hillock and a whisker develop over more than several micrometers, they may cause a serious problem in forming an integrated circuit in which wiring lines and elements are integrated at intervals of several micrometers.
As a method for preventing hillocks and whiskers, a very small amount of rare earth element, silicon, or some other element is mixed into aluminum. However, even with this method, hillocks and whiskers still occur when heated at a temperature higher than about 400.degree. C.
Further, as in the case of gate lines, it is increasingly required that aluminum wiring lines be formed at an early stage of a process. The problem of hillocks and whiskers is more serious in this case, because aluminum wiring lines are necessarily subjected for a longer time to heating steps and steps such as ion implantation which unavoidably involve heating.
Hillocks and whiskers are problematic because they may short-circuit wiring lines which are adjacent to each other vertically or horizontally. This problem becomes conspicuous as dimensions of design rules and wiring line pitches are reduced. In particular, if the wiring line pitch is shorter than 2 .mu.m, hillocks and whiskers in the lateral direction more likely cause short-circuiting between wiring lines adjacent to each other vertically or horizontally.
At a location where wiring lines cross each other, the upper wiring line needs to be formed over the lower wiring line through an interlayer insulating film (for instance, a silicon oxide film). In this case, if the interlayer insulating film does not provide proper step coverage, the upper wiring line may have a stepped disconnection or a local increase in resistance. Where an interlayer insulating film is deposited after formation of a wiring line made only or mainly of aluminum and then a second-layer wiring line is formed, hillocks and whiskers that unavoidably occur as described above deteriorate the step coverage of the interlayer insulating film. As a result, the second-layer wiring line, which is formed on the interlayer insulating film, has such problems as a stepped disconnection.
To solve this problem, a technique has been proposed in which an anodic oxide coating is formed on exposed surfaces of an aluminum wiring line by performing anodization with the aluminum wiring line used as the anode. For example, where a material made only or mainly of aluminum is used as a wiring material, hillocks and whiskers can be prevented by forming an oxide film of the material made only or mainly of aluminum on the top and side surfaces of a wiring line.
However, to perform anodization, it is necessary that a wiring line pattern which is different from that of an intended circuit be formed to allow currents to flow through all the wiring lines, and etching be conducted after the anodization to obtain an intended wiring line pattern. This procedure is not preferable because of an increased number of manufacturing steps. In particular, this re-patterning is not preferable in terms of production yield, because it likely causes etching of unnecessary portions since it is conducted after formation of the wiring lines of the intended circuit.
Further, as dimensions of design rules are reduced and wiring lines become thinner accordingly, stress which is imparted during anodization more frequently causes a defect mode where wiring lines are deformed or disconnected. This problem becomes conspicuous particularly in a case where wiring lines are complex in shape.
In addition, as dimensions of design rules are reduced and wiring lines become thinner accordingly, there come to appear influences of voltage drops in anodization which are caused by the resistance of wiring lines. That is, the voltage drops cause differences in the thickness of anodic oxide films formed.
This problem can be solved by making the cross-section of wiring lines larger than a necessary one, to thereby reduce voltage drops of the wiring lines during anodization. However, the increased cross-section of the wiring lines is an obstruction to increasing the degree of circuit integration.
The anodization technique can prevent hillocks and whiskers in forming wiring lines and electrodes made only or mainly of aluminum. However, on the other hand, it causes various problems as described above. Although there are conductive materials (for instance, tantalum) other than aluminum which can be anodized, the above problems still exist even with the use of such materials.